1. Field of the Invention
This invention relates to a linear DC brushless motor, and more particularly, to a linear DC brushless motor incorporating an armature core which can remarkably reduce detent thrust or drive force, in other words, can reduce such detent thrust caused by an end effect of its armature core attributable to its peculiar nature that the armature core of a linear motor must inevitably be composed of a linear core having a finite or limited length, namely having both ends in the direction of travel.
2. Prior Art
As linear motors have increasingly been applied to the office automation (OA) devices or to precision measuring instruments, various demands have been raised with respect to their servo control performance or precise positioning characteristics and as a consequence, linear DC brushless motors have attracted user's interest as one which is particularly suitable for such usage.
Generally, a linear DC brushless motor (hereinafter merely referred to LDM) is constructed based on such a manner that an ordinary DC brushless motor (or AC servomotor) is reformed into a linear shape. As its general construction of an LDM is shown in FIG. 7, movable element, namely, an armature core 1 is reciprocally movable right and left along its travel of motion shown by an arrow line in FIG. 7. The armature core 1 has a large number of teeth 1A or "comb teeth".
In the slots defined between successively adjacent teeth of the core, plurality of coils are fitted to make up a set of armature core in the movable element. In more detail, one of the legs of a preformed toroidal coil, say a first coil is inserted into one slot of the core between two teeth adjacent with each other expecting those at both ends in the direction of travel of the motor, the next leg, in a similar manner, to the next slot, and further leg(s) of the coil or a subsequent coil is inserted in the subsequent slot, one after another to make up a set of an armature core.
With regard to the stator, a large number of permanent magnets constituting a linear stator are arrayed on a plate of magnetic material to form an alternate magnetic pole arrangement as shown also in the drawing.
The relative position of the armature core with respect to the stator is detected by a sensor (not shown). Incorporated further in the LDM system are a control circuit 5 and a driver 6, the former controls the current to be supplied to each coil 2, responsive to the relative position of these two elements detected by the sensor, while the latter supplies the thus controlled current to the coils through a power feed line 7. There have been found undesirable phenomena in such type LDM, referred to "detent thrust" or "rippled thrust" attributable to the inherent magnetic pole construction of the stator, and one of the measures taken heretofore to reduce the detent thrust is such one as shown by FIG. 8, namely, permanent magnets 4 of the stator are arrayed parallel in skewed relation with respect to the length-wise axis of the stator 3.
It becomes possible to reduce the detent thrust caused by both the gaps or boundaries between adjacent magnetic poles and the teeth slots, by relying on the aforesaid skewed arraying of the stator permanent magnets, however, other problems of detent thrust due to the end effect of the armature core, that is, abrupt change in permeance from full value to almost zero, or viceversa, between the end touch and the exterior of the armature core, have not yet been solved.
These unsolved problems of the latter type are attributable to such inherent nature of the linear motor that the armature core which moves in itself or is movable relative to the stator has a finite length, namely, having both ends and this is the cause of the detent thrust as mentioned above which cannot be solved only by arraying the stator permanent magnets in a skewed manner as explained above.